Method of treating ferrous alloys



Patented Sept. 4, 1934 'ieizm 1.972.248 msrnon or mes-mm FEBROUS ALLOYS Cyril Stanley Smith, Cheshire, Conn, aaignor,

by mesne assignments, to Copper & Brass Research Association, New York, N. 1.. a corporation oi' New York No Drawing. Application April 5, 1982, Serial No. 603,448

16 Claims.

This invention relates to a method of heat treating certain alloys, and particularly alloy steels containing copper although it is not limited thereto.

It is an object of the invention to provide a process of heat treating such alloys to secure increased hardness and high tensile strength with no or only a slight decrease in ductility, and which process is simpler and can be carried out at much less cost than the old methods.

It has long been known that certain alloys are capable of being hardened by the process known as precipitation hardening. The hardening is due according to the current theories to the critical dispersion of a second phase throughout the crystal grains of the principal metal. The fundamental requirements are a difference in solubility of the second phase in the principal one so that at a sufficiently high temperature an 0 alloy of suitable composition will be rendered homogeneous. Cooling the alloy from this temperature at a sufiiciently rapid rate retains it at room temperature as a homogeneous solid solution, but on reheating to a suitable temperature less than the first the second phase will be precipitated and if the time is correctly chosen for the temperature employed its dispersion will be such as to produce considerable hardening.

The addition of copper in amounts approximating l per cent is known to confer on steels of medium and low carbon content the capacity for precipitation hardening when after cooling from a high temperature, for example about 850 C. to room temperature, they are reheated at temperatures in the neighborhood of 400 to 600 C.

The cooling from the high temperature treatment is customarily in air, although other means, for example water or oil quenching and even furnace cooling have been employed. The purpose of the cooling is to retain a substantial amount of the copper in solid solution so that on reheating to a temperature in the neighborhood of 500 C., the copper is thrown out of solution and the phenomenon known as precipitation hardening results due to dispersion of the copper particles throughout the iron. The cooling is sufliciently rapid to retain the copper in solid solution. If the alloy is cooled slowly there is some throwing of the copper out of solution and some precipitation hardening, but this throwing out of copper starts at a high temperature so is of such large particles as to have little hardening eiiect. If the precipitation treatment is carried out at 600 C., treatment for about 45 minutes suffices to produce maximum hardness and tensile strength, after which the hardness again decreases, while at 550 C. two hours. at- 500 C. four hours, and 450 C. thirty-two hours result in maximum hardness. One process for such treatment calls for heating the alloy steel to a tem- .0 perature of approximately 845 C. for a sumcient time to insure the entire mass being reached by the heat, air cooling the steel to room temperature and then reheating at a temperature of approximately 510 C. and again air cooling the 55 steel. The result of this treatment is to increase the hardness, yield point and tensile strength, while a slight decrease in elongation and reduction of area as indicated by the tensile test oc v curs. 7

I have found that the some improvement in properties may be obtained with a simpler and cheaper heat treatment consisting of cooling the steel, at a rate suflicient to retain a substantial amount of the copper in solid solution, directly to the temperature which in the above process is used for reheating without the intermediate cooling to room temperature. The process in its simplest form consists of heating the steel piece to be treated, which 'will contain from 0.5 to 5 percent of copper and have a carbon content up to a maximum of approximately 0.9 percent together with other elements present as the usual impurities or deoxidants or which may have been added for the purpose of modifying the precipitation of the copper or of increasing the basic tensile strength, to a temperature above 750 C. and below the melting point for sufficient time to dissolve a substantial amount of the copper; then air cooling the steel to a temperature between 400 C. and 600 C., for example approximately 500 C.; placing it in a furnace or otherwise maintaining it at a temperature between 400 and 600 C. for sufiicient length of time to obtain the desired improvement in phys- 9E ical properties and cooling it in air to room temperature. I have found that for substantially maximum hardening effect it should be maintained for about 4 hours at 500 C. If the temperature is increased less time is required, while if the temperature maintained is less than 500 C. the time is increased. The rate of cooling to the temperature for the precipitating treatment should be sufiiciently rapid to retain a substantial amount of the copper in solution. I have obtained excellent results by quenching directly into a salt bath held at the temperature to which it is desired to cool. Such rapid cooling is not, however, necessary, and in the case of the copper steels air cooling is sufliciently rapid. 11

while some further precipitation will occur on heating even after a normal furnace cooling, although the properties are somewhat inferior after such a treatment.

On account of the comparatively wide range of temperature during which precipitation will occur, it is possible to obtain precipitation hardening by a very slow cooling through the critical range. This may be achieved either by careful progressive lowering of temperature in a' furnace or by air cooling the pieces from the high temperature solution anneal to a temperature at the top of the precipitaion range and then placing the articles in a well insulated soaking pit where the temperature will not fall at too rapid a rate without external application of heat, unless the total mass of metal being treated is too small. However, greater control over the process can be obtained using the first method described above, in which the pieces to be treated are air cooled from the solution heat treatment approximately to the temperature of the precipitation anneal and then inserted in a furnace maintained at this temperature for the desired length of time. Final cooling to room temperature may be in the furnace, in air or by quenching, although air cooling will in general prove most satisfactory. The particular advantages of-my process lie in saving of time and fuel by the elimination of the reheating operation. Moreover, since the furnace for the precipitation heat treatment merely has to maintain the pieces being treated at the desired temperature and does not have to heat cold metal to this temperature, its capacity will be very much greater for a given size or for a given heat input. ,The process becomes increasingly valuable as the size of the treated part increases, and is particularly applicable to the treatment of rolled beams or other sections which can be run continuously through an air cooling stage or cooled by air blast, water spraying or partial quenching to about 500-600 C. and then run through a furnace at 500-600" C. for the precipitation treatment. If the rolling be finished at approximately 750 C. it is not necessary to reheat, but the rolled sections can be cooled to the precipitation temperature and treated directly. This is obviously a very simple process for heat treatment. A further advantage of the process lies in the uniformity of temperature through the -sections of the pieces being treated, for they enter the precipitation treating furnace hot and a long soaking period is not necessary.

Although the above discussion has been limited mainly to copper steels, the precipitation hardening also occurs in alloys with very low carbon content such as are known as wrought iron or 'ingot'iron with sufficient copper additions-indeed, it has been established that low carbon alloys harden to a greater extent than do the alloys containing medium or high carbon. The

method is also applicable to the treatment of alloys with or without carbon together with other elements, for example nickel, vanadium, manganese or chromium, in addition to those customarily added as deoxidizers or present as impurities. The copper content must, however, be greater than the solid solubility at a comparatively low temperature, for example 400 C., but the solubility must change appreciably with temperature so that a greater amount of copper will go into solution at the higher temperatures.

It is obvious to those skilled in the art that this method of directly heating for precipitation without intermediate cooling to room temperature can be applied to any alloys which can be precipitation hardened and which will require the precipitation treatment at a temperature higher than room temperatures. In-some cases, this would result in a less ductile alloy since the precipitate tends to form at the grain boundaries, but inrthe case of the copper steels and copper-iron alloys this effect does not appear.

In a series of experiments I have found that cooling somewhat below the temperature 01 precipitation is without substantial effect on the properties, that is, it does not prevent or interfere with the hardening effect, and for ease of handling it may prove advisable to allow the pieces to cool to somewhat below the precipitation temperatures and then bring them back to these temperatures. It is to be understood that any cooling to temperatures well above room temperatures, for example not below 200 C., is included in the present invention, for saving of heat by eliminating the extensive reheating required in the old method is still effected.

As an example of the properties obtainable, the

following will suffice to indicate the advantages of my process. These test results were obtained with a steel containing 1.06 percent copper and 0.19 percent carbon.

Having thus set forth the nature of my invention, what I claim is:

1. A process for treatment of iron alloys con- 12o taining up to a maximum of approximately 0.9 percent carbon and from 0.5 percent to 5.0 percentcopper, comprising heating the alloy to a temperature above 750 C. for a time sumcient to dissolve the major portion of the copper, cooling the alloy to a temperature between 400 and 000 C., maintaining it in this temperature range for a sumcient length of time to obtain a substantial improvement in tensile strength, and cooling in any manner to room temperature.

2. A process for treatment of iron alloys containing up to a maximum of approximately 0.9 percent carbon and from 0.5 percent to 5.0 percent copper, comprising heating the alloy at a temperature suflicient to dissolve the copper, cooling to a temperature of about 600 C., then cooling through part or all of the range 600-400 C. at a slow rate such that precipitation hardening is produced. and cooling in any manner to room temperature.

3. A process for treatment 0! iron alloys containing up to a maximum oi approximately 0.9 percent carbon and from 0.5 percent to 5.0 percent copper, comprising casting the molten steel into ingots, hot working these ingots, cooling the hot worked shapes directly to between 400 and 600 C. without intermediate cooling below about 400C., maintaining the steel at a temperature between 400 and 600 C. for a suiiicient length of 15 time to produce an increase in hardness, yield point and tensile strength, and cooling to room temperature.

4. A process for treatment of iron alloys containing up to a maximum of approximately 0.9 percent; carbon and from 0.5 percent to 5.0 percent copper, comprising casting the molten steel into ingots, hot working these ingots, cooling the hot worked shapes by any means to a temperature of about 600 C., then cooling through part or all of the temperature range 600-400 C. at a slow rate such that precipitation hardening is obtained, and cooling in any manner to room temperature.

5. A process for treatment of steels containing up to a maximum of approximately 0.9 percent carbon and from 0.5 percent to 5.0 percent copper, comprising heating the steel to a temperature above 750 C. for a time sufiicient to dissolve the major portion of the copper, cooling the steel to a temperature above 200 C. and below 600 C., maintaining it at a temperature between 400 C. and 600 C. for a suflicient length of time to obtain a substantial improvement in tensile properties, and cooling to room temperature.

6. A process for the treatment of iron alloys containing from 0.5 percent to 5.0 percent copper and up to .9% carbon, comprising heating the alloy at a suitable temperature for a suiflcient length of time to dissolve the copper, cooling to a temperature between 400 and 600 C. without intermediate cooling below 400 C., maintaining it in this temperature range for a suflicient length of time to obtain precipitation hardening, and cooling to room temperature. a

'7. A process for treatment of iron alloys containing up to a maximum of approximately 0.9 per cent carbon and from 0.5 per cent to 5.0 per cent copper, comprising heating the alloy to a temperature above 750 C. for a time sufllcient to dissolve the major portion of the copper, air cooling the alloy to a temperature between 400 and 600 C., maintaining it in thistemperature range for a suflicient length of time to obtain a substantial improvement in tensile strength, and then cooling to room temperature.

8. A process ior treatment of iron alloys containing up to a maximum of approximately 0.9 per cent carbon andfrom 0.5 per cent to 5.0 per cent copper, comprising heating the alloy at a temperature suflicient to dissolve the copper, air cooling to a temperature of about 600 C., then cooling through at least part of the range 600- 400 C. at a slow rate such that precipitation hardening is produced, and then cooling to room temperature.

9. A. process for treatment or iron alloys containing up to a maximum of approximately 0.9 per cent carbon and from 0.5 per cent to 5.0 per cent copper, comprising casting the molten steel into ingots, hot working these ingots, air

' cooling the hot worked shapes directly to between 400 and 600 C., without intermediate cooling below about 400 C., maintaining-the steel at a temperature between 400 and 600 C. for a sumcient length of time to produce an increase in hardness, yield point and tensile strength, and cooling to room temperature.

10. A process for treatment of iron alloys containing up to a maximum oi. approximately 0.9 per cent carbon and from 0.5 per cent to 5.0 per cent copper, comprising casting the molten steel into ingots, hot working these ingots, air cooling the hot worked shapes to a temperature of about 600 C., then cooling through at least part of the temperature range 600-400 C. at a slow rate such that precipitation hardening is obtained, and cooling to room temperature.

11. A process for treatment of steels containing up to a maximum of approximately 0.9 per cent carbon and from 0.5 per cent to 5.0 per cent copper, comprising heating the steel to a temperature above 750 C. for a time suflicient to dissolve the major portion of the copper, air cooling the steel to a temperature above 200 C., and below 600 C., maintaining it at a temperature between 400 C. and 600 C. for a sufiicient length of time to obtain a substantial improvement in zensile properties, and cooling to room temperaure.

12. A process for the treatment of iron alloys containing from 0.5 per cent to 5.0 per cent copper and up to .9% carbon, comprising heating the alloy at a suitable temperature for a sufiicient length of time to dissolve the copper, air cooling to a temperature between 400 and 600 C. without intermediate cooling below 400 C., maintaining it in this temperature range for a suiiicient length of time to obtain precipitation hardening, and cooling to room temperature.

13. The process which comprises heating a steel alloy containing between .5% and 5% copper to a temperature suflicient to dissolve substantially all of the copper, cooling the alloy to a temperature between 400 C.and 600 C.,maintaining it within said temperature range for a period of time sufficient to increase the hardness of the alloy and cooling the alloy to room temperature.

14. The method of making steel which comprises alloying therewith from 0.50% to 5.00% copper, cooling the steel at a rate such as to maintain the major portion of the copper in solution and until a temperature between 600 and 400 C. is reached, and maintaining the steel at a selected temperature within said temperature range for a period of time ranging up to 32 hours, the interval of time within said time period being dependent upon said selected temperature, to thereby eii'ect precipitation hardening of the copper steel.

15. The method of making steel containing from 0.50% to 1.00% carbon which comprises alloying therewith from 0.50% to 5.00% copper, cooling the steel at a rate such as to maintain the major portion of copper in solution and until a temperature between 600 and 400 C. is reached, and maintaining the steel at a selected temperature within said temperature range for a period of time ranging up to 32 hours, the interval of time within said time period being dependent upon said selected temperature, to thereby eflfect precipitation hardening of the copper steel.

16. The method of making steel containing from 0.50% to .90% carbon which comprises alloying therewith from 0.50% to 5.00% copper, heating the steel to a temperature and for a sufllcient time to bring the copper into solution, 

